Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries. Among these secondary batteries, lithium secondary batteries having high energy density and output voltage, long cycle life and low self-discharge ratio are commercially available and widely used.
Recently, increased concern over environmental issues has brought about a great deal of research associated with electric vehicles (EV) and hybrid electric vehicles (HEV) as substitutes for vehicles using fossil fuels, such as gasoline vehicles and diesel vehicles, which are a major cause of air pollution. Although nickel metal hydride (Ni-MH) secondary batteries have mostly been used as a power source of such EV and/or HEV, a great deal of studies into use of lithium secondary batteries having high energy density and high discharge voltage are underway and some of these are commercially available.
Specifically, the lithium secondary battery used for EV must have high energy density and high output power in a short time and, in addition, be used even under severe ambient conditions for at least 10 years. Therefore, it is necessary for the above secondary battery to have excellent safety and long lifespan, as compared to existing small lithium secondary batteries.
A lithium ion secondary battery used as conventional small battery includes a lithium-cobalt composite oxide having a layered structure in a cathode and a graphite material in an anode. However, for such lithium-cobalt composite oxide, although cobalt is widely used as a main component, this material encounters disadvantages including, for example, high cost due to scarcity of cobalt, low safety, etc., thus having limitations in use as a power source in EV applications, and the like. As a cathode used in a lithium ion battery for EV, a spinel structure lithium manganese composite oxide comprising manganese, which is relatively cheap and has excellent safety, may be suitably used.
However, for lithium manganese composite oxides, manganese is dissolved in an electrolyte when the electrolyte is stored at a high temperature, in turn deteriorating characteristics of the battery. Therefore, improved techniques to prevent such deterioration in battery characteristics are required. In addition, compared to typical lithium cobalt composite oxides or lithium nickel composite oxides, the lithium manganese composite oxide has low capacity per unit mass, in turn causing limitation in capacity increase per unit mass of battery. Therefore, there is also a requirement for development and/or design of a novel battery to solve the foregoing problems, ultimately enabling utilization thereof as a power source for EV.
In order to overcome various weaknesses described above, a great deal of studies into fabrication of electrodes using mixed cathode active materials have recently been conducted. For instance, Japanese Laid-Open Patent Publications Nos. 2002-110253 and 2004-134245 disclose techniques for using mixtures of lithium manganese composite oxides and lithium nickel cobalt manganese composite oxides in order to increase regenerative output or the like. However, the lithium manganese oxide still entails some disadvantages such as poor cycle life and limitations in improving safety.
Meanwhile, in order to increase capacity and lifespan and/or improve high rate discharge characteristics of a battery, Korean Patent No. 0570417 discloses use of a spinel lithium manganese oxide (LiMn2O4) as a cathode active material; Japanese Laid-Open Patent Publication No. 2002-0080448 discloses use of a cathode active material containing lithium manganese composite oxide; and Japanese Laid-Open Patent Publication No. 2004-134245 discloses use of a cathode active material including a spinel lithium manganese composite oxide as well as a lithium transition metal composite oxide and, in addition, secondary batteries have been fabricated using the foregoing cathode active materials, respectively.
Secondary batteries used for EVs, HEVs, etc. generally require excellent rate properties and power characteristics depending upon vehicle operating conditions.
However, conventional technologies in the prior art have not yet proposed specific configurations of secondary batteries having sufficient lifespan and safety.